A competitive radioimmunoassay (RIA) for a tumor associated antigen (termed TAG-72) identified by monoclonal antibody B72.3 has been established. The distribution of TAG-72 in human tissues has been shown to be highly specific for carcinomas with no significant reactivity to normal tissues. The RIA was used to examine sera from patients with colorectal carcinomas, other malignancies and normal sera. A mean of 2.2 units/ml of TAG-72 was found in the normal sera. When a cut-off level of 3 standard deviations above the mean level of TAG-72 found in normals is used, no patient with inflammatory disease or other benign colon diseases exhibited abnormal levels of TAG-72. Thirty-five percent of sera from advanced colon cancer patients and patients with other carcinomas were positive for TAG-72. No increase in TAG-72 reactivity was seen in normal sera or that from patients with melanomas or sarcomas. Comparison of the TAG-72 levels in sera with antigens recognized by the monoclonal antibodies currently used to screen sera of carcinoma patients clearly demonstrated that TAG-72 is different from the other antigens, and that TAG-72 can be found in some sera where no antigen is detected by the commercially available MAb RIAs. TAG-72 has been purified from extracts of human colon carcinoma xenograft in athymic mice using molecular sieving and antibody affinity chromatography. It has an apparent molecular weight of greater than 1 million daltons, and has been characterized as a mucin.